Contrast
This grid, known as Hermann's Grid, is an example of how contrast affects color perception. The area at the corners of the black boxes appear gray. This happens because of something called lateral inhibition. In the retina when some light-receiving cells are activated others around them shut down.

You will notice that where the white lines intersect, there is black on four sides, whereas the lines themselves are surrounded by black on only two sides. When you look at the intersections, the cells in the retina are surrounded on four sides by other cells that are also receiving light. They are therefore more inhibited than the cells focused on the lines. It is their inhibition that causes the dark spots to appear.